Several applications for a compact battery having a high power to energy ratio are apparent. The use for such a battery is of particular interest where space and weight limitations are of paramount importance. Such is the case in the transportation industry, and more specifically in the automotive industry. A bipolar battery, such as a lithium/iron disulfide battery, seems well suited for this application because of its relatively compact configuration.
As is well known to those skilled in the pertinent art, the operation of a chemical battery requires an anode, a cathode and an electrolyte which is positioned between the anode and cathode. Further, it is necessary that the anode current collector of the battery be electrically insulated from the cathode current collector, and that the electrolyte be contained between the anode and cathode. An effective seal must accomplish both of these requirements.
Several factors must be considered in the manufacture of a bipolar battery seal in order for the seal to satisfy the different requirements. First, because the seal must insulate the current collectors from each other, the seal must contain a dielectric material, such as a ceramic or a glass. Second, since the current collectors must be conductive, and are therefore typically made of a metal, the dielectric must somehow be effectively bonded to the metallic current conductors. Third, the seal must be hermetic so that electrolyte between the anode and cathode will not leak from the battery cell.
One approach to the manufacture of a hermetic seal for a bipolar battery involves creating a "graded" transition zone between the ceramic part of the seal and the metallic current collectors. More specifically, graded seals include variously proportioned mixtures of ceramic and metallic constituents in the transition zone. The intent, of course, is to have a greater concentration of ceramic constituents in the transition zone near the ceramic, and a greater concentration of metallic constituents in the transition zone near the metallic conductors. Such structures accommodate the differences in thermal expansion between the ceramic and the metallic constituents. Such an approach was first suggested in U.S. Pat. No. 3,324,543 which issued to C. I. McVey et al for an invention entitled "Pressure Bonded Ceramic-To-Metal Gradient Seals". Others have followed McVey et al. For example, H. Gibson, General Electric Co., presented a paper at the 1967 Thermionic Conversion Specialist Conference in Palo Alto, Calif., entitled "Fabrication and Evaluation of Graded Type Ceramic-To-Metal Seals" wherein a graded seal was considered in detail. The seal disclosed by McVey et al, and evaluated by Gibson, however, required a prefabricated ceramic such as Lucalox (Al.sub.2 0.sub.3).
The present invention recognizes that graded seals which are manufactured using a prefabricated ceramic are undesirably bulky, unnecessarily more complicated to manufacture, and more expensive. As appreciated by the present invention, the prefabrication of a ceramic core to achieve the required dielectric and hermetic properties of an effective bipolar battery cell can be circumvented.
In light of the above it is an object of the present invention to provide a seal for a bipolar battery which is monolithic and manufacturable with only one sintering operation. Another object of the present invention is to provide a seal for a bipolar battery which will effectively withstand significant temperature differentials during manufacture and operational installation. Yet another object of the present invention is to provide a seal having diminished dimensions for the manufacture of individual battery cells which, when joined together, make a battery having a compact configuration. Still another object of the present invention is to provide a seal for a bipolar battery which can be mass produced. Another object of the present invention is to provide a seal for a bipolar battery which is relatively simple to manufacture and comparatively cost effective.